crypto: talitos - fix aead sglen for case 'dst != src'

For aead case when source and destination buffers are different,
there is an incorrect assumption that the source length includes the ICV
length. Fix this, since it leads to an oops when using sg_count() to
find the number of nents in the scatterlist:

Unable to handle kernel paging request for data at address 0x00000004
Faulting instruction address: 0xf2265a28
Oops: Kernel access of bad area, sig: 11 [#1]
SMP NR_CPUS=8 P2020 RDB
Modules linked in: talitos(+)
CPU: 1 PID: 2187 Comm: cryptomgr_test Not tainted 3.11.0 #12
task: c4e72e20 ti: ef634000 task.ti: ef634000
NIP: f2265a28 LR: f2266ad8 CTR: c000c900
REGS: ef635bb0 TRAP: 0300   Not tainted  (3.11.0)
MSR: 00029000 <CE,EE,ME>  CR: 42042084  XER: 00000000
DEAR: 00000004, ESR: 00000000

GPR00: f2266e10 ef635c60 c4e72e20 00000001 00000014 ef635c69 00000001 c11f3082
GPR08: 00000010 00000000 00000002 2f635d58 22044084 00000000 00000000 c0755c80
GPR16: c4bf1000 ef784000 00000000 00000000 00000020 00000014...